1. Field of the Invention
The present invention relates to an improvement of a power steering system which is adapted to control a flow quantity of an operating oil introduced into a steering output section in accordance with driving conditions of a vehicle.
2. Description of Related Art
An example of this type of conventional power steering system has been disclosed in Japanese Unexamined Patent Publication No. 2001-163233A, which is provided with a pump driven by an engine to discharge an operating oil, and a flow control valve controlling a flow quantity QP of the operating oil introduced from the pump to the steering output section.
The flow control valve is provided with a solenoid valve controlled by a controller, and a spool valve to return a part of the operating oil toward a tank side in response to a differential pressure of the operating oil between after and before flowing through the solenoid valve where the flow control valve controls the flow quantity QP of the operating oil supplied to the steering output section. Controlling the flow quantity QP of the operating oil supplied to the steering output section to be close to a required target flow quantity QM prevents an extra operating oil from being supplied to the steering output section, thus reducing a loss in driving force of the pump.
The controller receives a signal of a steering angle θ from a steering angle sensor and a signal of a vehicle speed V from a vehicle speed sensor, calculates a steering speed ω by differentiating the steering angle θ, and estimates a required target flow quantity QM based on the steering angle θ, the steering speed ω, and the vehicle speed V, thereby to control operations of the flow control valve.
Since the controller is configured to estimate the target flow quantity QM without using a detected value of steering torque, a torque sensor for direct detection of the steering torque is not required, resultantly leading to reduction in the cost of the control system.